In wireless mobile systems, frequency reuse is an important technique to improve the overall system capacity by reusing the scarce radio spectrum resource. Improvement of the system capacity, however, is achieved at the cost of link performance due to increased interference. In a cellular orthogonal frequency division multiple access (OFDMA) system, there is no intra-cell interference as users remain orthogonal. Reusing the radio spectrum, however, will result in inter-cell interference because the same frequency band is reused by base stations serving neighboring cells.
FIG. 1 (prior art) is a diagram that illustrates a cell structure of a cellular OFDMA system 1. Cellular OFDMA system 1 includes a cell structure having a frequency reuse factor 1/K equal to ¼. Frequency reuse factor 1/K represents the number of cells that cannot share the same frequency bands for transmission. In the example of FIG. 1, the entire licensed spectrum is partitioned into four frequency bands, and every four neighboring cells form a cluster of four cells, within which each cell is served by a different frequency band. In one example, base station BS4 and base station BS5 share the same frequency band #1 to serve mobile station MS6 located within cell 2 and to serve mobile station MS7 located within cell 3 respectively. As a result, when BS4 transmits a desired data signal to communicate with MS6, it also transmits an undesired interference signal to MS7. Such interference signal reduces the signal to interference-plus-noise ratio (SINR) of mobile station MS7 and thus reduces overall quality of service. Although a smaller frequency reuse factor 1/K generally results a larger separation (e.g., SQRT (3K)* R, where R is the cell radius) from interfering sources, the available radio resource in each cell becomes lower (e.g., 1/K of the licensed spectrum).
Other techniques such as fractional frequency reuse (FFR) have been proposed for cellular OFDMA systems to achieve a better tradeoff between system capacity and quality of service. FIG. 2 (prior art) is a diagram that illustrates FFR in a cellular OFDMA system 10. Cellular OFDMA system 10 includes a cell 11 that is partitioned into cell region 1 and cell region 2. Cell region 1 is located in a geographic area closer to serving base station BS12 while cell region 2 is located in a geographic area further to serving base station BS12. In addition, the radio spectrum of OFDMA system 10 is partitioned into a first frame zone and a second frame zone in time domain. Under adaptive frequency reuse technique, different frame zones are applied with different frequency reuse factors to serve mobile stations located in different cell regions. In the example of FIG. 2, the first frame zone has a higher frequency reuse factor of 1/K=1 to serve cell region 1 while the second frame zone has a lower frequency reuse factor of 1/K=⅓ to serve cell region 2. Mobile station MS17 located in cell region 1 is therefore served by BS12 through the first frame zone with 1/K=1, and mobile station MS18 located in cell region 2 is therefore served by BS12 through the second frame zone with 1/K=⅓. Because mobile station MS17 is located close to the center of cell 11, it is presumed to receive relatively strong data signals from BS12 and relatively weak interference signals from neighboring interfering sources. On the other hand, because mobile station MS18 is located close to the boundary of cell 11, it is presumed to receive relatively weak data signals from BS12 and relatively strong interference signals from neighboring interfering sources. Therefore, by serving MS1 using a higher reuse factor (1/K) and serving MS2 using a lower reuse factor (1/K), a good tradeoff between system capacity and quality of service is achieved.
Unfortunately, FFR technique based on geographic locations is not always effective. As illustrated in FIG. 2, a physical structure 14 is located between mobile station MS18 and an interfering base station BS13. Interfering base station BS13 thus transmits relatively strong interference signal 15 to MS17 and relatively weak interference signal 16 to MS18. Under the existing frequency reuse pattern based on cell regions illustrated above, MS17 located in cell region 1 suffers strong interference from BS13 and yet is served with a higher 1/K=1, while MS18 located in cell region 2 enjoys good quality of service and yet is served with a lower 1/K=⅓. Therefore, FFR technique based on geographic locations is not suitable under dynamic network conditions. There remains a challenge to be able to dynamically measure interference, determine frequency reuse patterns, and configure radio resource allocation such that link performance and system capacity in a wireless mobile system remain well balanced.
Interference measurement mechanisms have been addressed in various wireless mobile systems. For example, in traditional cellular FDMA (e.g. GSM) or CDMA systems, narrow band signals are transmitted and received by transceivers. Due to the narrowband characteristic, an FDMA system can only measure the signal power or interference over one single time-frequency region at a given time. Such FDMA system is not able to freely measure among different time-frequency regions because the RF center frequency of the FDMA system needs to be adjusted accordingly. In contrast, in an OFDMA system, wideband signals are transmitted and received by transceivers equipped with Fast Fourier Transfer (FFT) functionality. Such OFDMA system allows signals to be easily transmitted and received over any specific time-frequency region among wider channel bandwidth. Therefore, the transceivers of the OFDMA system can freely measure the signal power or interference over a time-frequency region different from the time-frequency region for data receiving without changing the RF center frequency. This is a distinct feature of OFDMA systems as compared to other traditional cellular FDMA or CDMA systems.